1. Field of the Invention
This invention relates to a method for the production of 2,3,4,5-tetrafluorobenzoic acid. More particularly, this invention relates to a novel method for producing 2,3,4,5-tetrafluorobenzoic acid in a high yield by decarboxylating 3,4,5,6-tetrafluorophthalic acid in an aqueous medium.
2. Description of Prior Art
2,3,4,5-Tetrafluorobenzoic acid is a compound useful as an intermediate for medicines and agricultural pesticides. Numerous techniques have been proposed to date for the decarboxylation of phthalic acid derivatives in an aqueous medium [such as in Chemical Abstracts 41, 2083d-(1947) and U.S. Pat. No. 1,939,212, for example]. These techniques are invariably aimed at obtaining unsubstituted benzoic acid. This invention effects as does none of the aforementioned conventional techniques efficient decarboxylation of 3,4,5,6-tetrafluorophthalic acid which is a phthalic acid substituted by fluorine atoms. It has been ascertained to us that when any of the conventional techniques is applied without modification to this invention, the reaction gives rise to secondary products in large amounts and fails to produce 2,3,4,5-tetrafluorobenzoic acid in a sufficient yield.
Generally, a very few methods have been recognized as useful for the production of halogenated benzoic acids by the decarboxylation of phthalic acids substituted by halogens. Undeniably, U.S. Pat. No. 2,439,237 discloses effective production of 2,3,4,5-tetrachlorobenzoic acid by heating 3,4,5,6-tetrachlorophthalic anhydride in an alkaline aqueous solution under application of pressure at a temperature of 220.degree. to 280.degree. C. It, however, makes no mention about fluorides.
We have studied the method of the U.S. Pat. No. 2,439,237 to determine whether or not it can be applied to 3,4,5,6-tetrafluorophthalic acid which is the starting material for the present invention. As indicated in a comparative experiment (control), 3,4,5,6-tetrafluorophthalic acid was simply heated in an alkaline aqueous solution in accordance with the method of the U.S. Patent for the purpose of decarbonating the acid. The reaction which ensued mainly gave rise to trifluorophenol having fluorine atoms substituted by hydroxyl groups and failed to produce 2,3,4,5-tetrafluorobenzoic acid selectively. It may be safely concluded that the fluorine atom at the para position of a benzene ring posessing such an electron-attracting group as the --COOH group is more susceptible to the nucleophilic substitution reaction than the chlorine atom at the same position. By the method of the U.S. Pat. No. 2,439,237, therefore, the alkaline substance is believed to form a phenol by substitution of the fluorine atom. It is, accordingly, proper to conclude that the method, when applied to 3,4,5,6-tetrafluorophthalic acid, is liable to induce secondary reactions and is not useful for the decarboxylation aimed at by the present invention. Methods for the decarboxylation of 3,4,5,6-tetrafluorophthalic acid are described in G. G. Yacobson et al, Journal of General Chemistry, 36, No. 1,144 (1966) and British Pat. No. 2,122,190. The former method causes this acid to react in dimethylformamide as a solvent at a temperature of 145.degree. C. This method, however, dose not deserve commercialization because it produce 2,3,4,5-tetrafluorobenzoic acid in low a yield of 44.0 mol %. The latter method causes the acid to react in an organic solvent, and that at a temperature of 200.degree. C. It does not produce any 2,3,4,5-tetrafluorobenzoic acid but barely gives rise to 1,2,3,4,-tetrafluorobenzene in a yield of 0.5%.
An object of this invention, therefore, is to provide a novel method for the production of 2,3,4,5-tetrafluorobenzoic acid.
Another object of this invention is to provide a novel method for producing 2,3,4,5-tetrafluorobenzoic acid in a high yield by the decarboxylation of 3,4,5,6-tetrafluorophthalic acid in an aqueous medium.